There's a very simple way to save money on petrol: slow down. A car speeding at 80mph uses 10% more fuel than one cruising at 70mph, according to the Department for Transport.

This week, Spain introduced a 110km per hour (68mph) limit on its motorways as part of a series of measures designed to cut petrol consumption in the country by more than 5%. Yet Britain seems to be heading in the opposite direction, with transport secretary Philip Hammond suggesting recently that our speed limit could be raised to 80mph to put the economy in the fast lane.

But while Clarkson-ite petrolheads might cheer a rise in speed limits, in practice British drivers are already taking their foot off the pedal to save cash. Of 15,000 drivers who took part in a recent AA survey, 59% said they would think about slowing down if it saved them money. During the last petrol price spike in 2008, average speeds on British motorways fell to 69mph from 70mph the year before, while the number exceeding the limit by more than 10mph fell even more. But once petrol prices eased back again, average speeds rose.

But you can drive at the same speed and still save on petrol by sticking to a few simple techniques, according to automotive engineer Anthony Sale of Powertrain.

• Avoid over-revving

Drivers typically let the revs run to 3,000 per minute on a petrol car and 2,500 on a diesel before moving up a gear, but Sale says we should be changing up when we hit 2,500 revs on a petrol and 2,000 on a diesel, and move into fifth (and sixth on new models) at the appropriate point.

Your engine may not sound as if it is labouring, but in reality speeds above 75mph-80mph are when your engine starts glugging gas. "At very high speeds engines will rev higher, the engine will have to work much harder and it will start to drink far more petrol," Sale says.

In general, the economical band for petrol vehicles is between 1,500 and 2,500 rpm (for diesels it is between 1,300 rpm and 2,000 rpm), and these bands are the same for the vast majority of vehicles, Sale says. "The exception to the rules are vehicles which have either a very high or a very low power to weight ratio. If the engine has a lot of power but very little weight (for example, the 1.6l Mini diesel) then the revs can be kept closer to 1,600 or 1,700 rpm rather than going right to 2,000 rpm. Equally, if a low-powered engine is fitted to a heavy vehicle then it is likely that the engine will have to work much harder to move the mass of vehicle along, hence the need to rev the engine higher than the 2,500rpm top end of the band.

"It is important to remember that at whatever point you change gear you don't want the engine to labour as you tread back in."

• Acceleration and deceleration

"Stamping on the brakes and then accelerating hard is efficient driving's worst enemy. An efficient driver is a smooth driver," Sale says. If you have cruise control, try to use it whenever appropriate."

There are times when you can cut your fuel usage to zero: by cruising in gear as you slow down to a junction or roundabout. Many drivers occasionally put the car into neutral and let it coast as it slows, which is a no-no. "Lifting off the throttle, rather than putting it in neutral, means you will use no fuel at all," Sale says. It's safer, too.

• Speed bumps

Braking hard, accelerating, then braking hard for the next speed bump means you drink petrol, Sale warns. Drive along speed-humped roads at a steady 15mph-20mph instead.

• Tyres

Over time, tyres will naturally leak a bit of air. The RAC says you can improve fuel consumption by up to 2% if you regularly check and maintain pressures. It's safer, too: dozens of people a year die due to poorly-inflated tyres.

• Roof racks

A report by car manual maker Haynes says get rid of the rack – fully loaded it can add 30% to fuel consumption. But AutoExpress says that's a myth. Its test found that those with aerodynamic designs added little to the fuel bill.

But reducing weight – for instance by clearing unnecessary items from the boot – will definitely make a difference, albeit small, to fuel consumption.

• Air con v open windows

At low speeds, such as driving around town, air conditioning can add 5%-7% to fuel costs, Sale says, so just wind the window down. At higher speeds the effects are less noticeable.

• Radio/CD player

In a survey by BP, 10% of drivers thought that turning off the radio improved fuel consumption. It doesn't.

• Engine size

Don't assume smaller engines use less petrol. A big car with a 2l engine may use less than the same car with a 1.6l engine as it strains to pull a larger weight. On test, a people carrier with a small engine recorded worse fuel efficiency than one with a bigger engine.

• Petrol v diesel

In general diesels will use less fuel, but as they typically cost more to buy the savings aren't always obvious. The Environmental Transport Association has a calculator on its website which gives you an indication of the costs of running on diesel, petrol, biodiesel, electric battery or fuel cells. Sale says: "I've driven diesels with dreadful fuel efficiency, and petrol cars that have excellent fuel efficiency."

• Learning to drive more smartly

The AA Driving School offers a Drive Smart deal for already qualified drivers wanting to cut petrol consumption. It consists of a minimum of two hours of in-car lessons, and the AA says "pupils have typically been able to cut 10% from their fuel consumption (equating to savings of £160 a year based on today's petrol price)."